Emission material changing global lighting?
In your render settings, your Light Paths options are configured to some very low values:

Compare these to the default values:

In particular, your Clamping settings are highly unusual. The default value for Direct Light is 0.00, which disables clamping altogether, but you have it set to 0.63, which is quite small. This essentially means your entire scene will be unusually dark, since the contribution of your direct lighting is artificially clamped.
Without the Emission providing an additional light source, Blender’s color management largely masks the effect of the clamping. Since the scene is uniformly dark, the brightest point is still the background, which will be chosen as the scene’s white point, scaling everything back to what is roughly normal.
But when you add the additional light source, that stops being true. Your sphere becomes the brightest object in the scene, which shifts the white point upward, above the value reflected by your background. Consequently, the background appears darker in the final composited image.
Resetting all the Light Paths options to their default values fixes the issue, so the background is consistently exposed regardless of the strength of the emission:


(Note also that the glass sphere looks much better with these settings, a result of raising the number of transmission bounces.)